The ever-increasing importance of voltage-controlled oscillators (VCOs) in RF devices for the telecommunications field, including 900 MHz telephones, cellular telephones, DECT telephones, RF data links, etc., presents a growing need to reduce the effect of unwanted oscillations due to local resonance, i.e. package parasitics, without reducing the gain of the circuit. At higher operating frequencies, parasitics are on the same order as the resonant tank components and therefore, the impact of package parasitics become more pronounced and more of a concern. In fact, parasitics may significantly affect the operation of a VCO.
FIG. 1 shows a conceptual block diagram of a voltage controlled oscillator consisting of a passive resonant circuit 10, typically on a printed wiring board, connected to an active circuit 20, that is part of an integrated circuit, through package leads which produce parasitic resonance 30. A requirement for oscillation is that active circuit 20 must make up for the losses in the passive resonator 10, which requires that the circuit provide a negative resistance at the resonant frequency of passive resonator 10. The negative resistance is often formed with one or more separate feedback paths into the circuit. Feedback path 40 is illustrated. This negative impedance is usually broadband, and therefore, the parasitic resonance 30 of the package leads can cause the circuit to oscillate at an undesired frequency.
In an attempt to address the concern of undesired oscillation modes due to package parasitics, previous Radio Frequency Voltage Controlled Oscillator (RF VCO) designs have relied on making the active circuit sufficiently narrow band such that it only oscillates at the desired frequency, or adding damping resistors to the leads to make the Q of the parasitic resonance much lower than the Q of the passive resonator. In fact, these techniques may be effective, provided that the passive resonator has a very high Q factor. However these techniques do not provide a broad solution to the problem of package parasitics in VCOs.
Previous designs have also essentially restricted package types and lead assignments to those that minimize lead and bond wire inductance. In addition, in various designs the capacitance to VSS on the pads (PAD1 and PAD2) has been minimized to maintain the unwanted resonant frequency as far above the desired frequency of oscillation as possible, again lowering the likelihood of unwanted oscillation. Finally, electrostatic discharge (ESD) protection diodes are also typically removed from the pads (PAD1 and PAD2) to minimize the capacitance to the VSS. While each of these restrictions help to address the effect package parasitics on VCOs having external tank circuits, the requirements of VCO's in various RF devices do not always tolerate such restrictions.
Finally, other attempts to address the problems associated with package parasitics include designing a VCO having an "internal" LC tank (as described for example in IEEE Journal of Solid-State Circuits, Vol. 32, No. 5, May 1997, p. 736), which uses integrated planar inductors in the resonance LC-tank, so that external elements are unnecessary, i.e., thereby removing the bond wires and leads from the resonant circuit path and reducing package parasitics. However, for good phase noise performance, the resonant circuit should have a large Q; currently sufficiently large Q on-chip inductors are not available. In addition, this technique requires high Q on-chip varactors, which also currently do not exist. Accordingly, the phase-noise performance of on-chip oscillators is limited by the Q of the resonating elements available on-chip, again giving rise to poor performance of the VCO.